I find that if it says 20 minutes, it usually takes me around 25 minutes. But that is primarily because of what what I add to the message from the pulpit. If I stuck just to the content, it would probably be fairly accurate.

Without easily made adjustments for personal style, this is a largely symbolic feature. I find that I can READ a 3,000 word manuscript in about 20 minutes, but it takes me 35-40 to PREACH that same manuscript, because I don't read it word for word, but use it as a map for the sermon.

Thank you to each who has responded. It confirms that the timer is like the MPG (MPL in the metric world?) on a vehicle sticker. It always has the disclaimer "Your actual mileage may vary"

Honestly, my final product in the Sermon Guide is a "detailed outline" more than a "manuscript". I am convinced that a preacher must be prepared in heart and mind before preaching, but that the act of preaching is a human/divine exercise if it is to be transformative so I don't fret if I go off script. I would guess this explains the variance.

So to the OP of the thread. I don't know how a computer could calculate a more accurate timer for many of us. I guess experience will teach the homiletician to mentally adjust the data to his/her personal situation.